1. Field of the Invention
The present invention relates to cold spraying methods and apparatus in which powder in a gas flow is sprayed under pressure onto a workpiece at or close to the ambient temperature, to form a coating of the powder on the workpiece.
2. History of the Prior Art
It is well known in the art to form coatings of metals or other materials by spraying a powder or other particulate form of the material using a plasma system. Plasma systems spray the particulate material through a nozzle located within a plasma chamber, under very high temperatures and high pressures. The pressures combine with vacuum pumps or other sources of low pressure downstream of the plasma chamber to form a plasma flame. The powder or other particulate matter which is introduced into or close to the nozzle is heated to melt or near melt and forms a part of the flame. The plasma flame carries the molten material to a workpiece located downstream of the nozzle within the plasma chamber, where a dense coating of the material is formed on the workpiece. Such plasma systems have found widespread use for certain applications such as the refurbishment of aircraft engine parts, where a dense coating of metal or other material must be formed on the parts. An example of such systems is provided by U.S. Pat. No. 5,225,655 of Muehlberger, which issued Jul. 6, 1993.
Because of the extreme conditions under which plasma systems operate, they are typically expensive to build and consume considerable space. Consequently, less expensive and more compact systems have been investigated.
One alternative system which has gained favor for certain applications is the so-called cold spray system. Cold spray systems introduce a gas such as an inert gas under pressure into a cold spray gun. The powder or other particulate to be sprayed is also introduced into the cold spray gun where it mixes with the pressurized gas for eventual discharge from the gun, such as through a spray nozzle. The gas is sometimes heated to a desired extent, and the powder is often introduced into the heated gas at a point where it is also subjected to a desired amount of heating. The mixture of gas and powder exits the cold spray gun under pressure and is sprayed onto an adjacent workpiece to form the desired coating thereon. By definition, the gas which has exited the cold spray gun is relatively cool, in cold spray systems. Typically, the gas is at or close to the ambient temperature outside of the cold spray gun. While the powder is typically heated to some extent (but not to the extent that oxidation occurs), it is not heated to melt as in the case of plasma systems nor is it even heated to the softening point of the powder. Nevertheless, the temperatures and pressures which are present as the spraying occurs combine to form a relatively dense coating of the material of the powder on the workpiece. An example of a conventional cold spray system is provided by U.S. Pat. No. 5,302,414 of Alkhimov et al., which issued Apr. 12, 1994.
Cold spray processes provide certain advantages over plasma systems, beyond the fact that they are more compact and less expensive. Such advantages relate to the relatively cool temperatures of the spray and the fact that the powder particles are not molten. Molten powder tends to coat and sometimes clog various parts, passages and orifices which are not intended to be coated with the powder material. This creates a maintenance problem for the equipment, and in some cases greatly shortens the life span thereof. Also, cold spraying is better for certain compounds which are affected by high heat and oxidation.
While conventional cold spray processes are suitable for many applications, there is room for improvement. One area has to do with the density and uniformity of the coatings created on the workpiece. Because of the relatively low temperatures and the relatively low pressure of the spray directed onto the workpiece, the coating formed on the workpiece may have less than desirable or acceptable density or uniformity for certain applications. Also, it would be desirable to provide a spray system with greater versatility so that heating of the gas and of the powder particles within the cold spray gun can be varied relative to one another to optimize conditions. A still further area of possible improvement relates to conservation of the inert gases typically used in such systems. The inert gases such as helium which are often used in such systems tend to be relatively expensive. Consequently, it would be desirable to be able to conserve on the amount of new gas which must be introduced into the system for various spraying operations.
Briefly stated, the present invention provides improved methods and apparatus for cold spraying. In particular, the present invention provides for low pressure cold spraying methods and apparatus which are highly advantageous over conventional cold spraying methods and apparatus. To accomplish this, the cold spray is introduced into an ambient pressure which is substantially less than atmospheric pressure. This results in substantial acceleration of the gas and included powder particles or other particulate exiting the cold spray gun, with the result that denser and more uniform coatings are formed on the workpiece.
In accordance with a further aspect of the invention, gas and powder mixture from the workpiece is filtered before being fed to a compressor which compresses the inert gas. The compressed inert gas is then recycled to the source of such gas for reuse in subsequent cold spraying operations. This results in the realization of considerable savings in the amount of expensive inert gas which is often used for best results.
In accordance with a still further aspect of the invention, the gas is fed through a heating coil within the cold spray gun for heating of the gas by a certain amount prior to exiting through a nozzle at the end of the gun. At the same time, an arrangement of valves and injection points at various locations along the heating coil and within the nozzle enable powder to be introduced at a selected one of a plurality of different locations along the heating coil and within the nozzle. In this manner, heating of the powder and of the gas can be varied relative to each other to achieve optimal results.
In a cold spraying method according to the invention, a spraying orifice is provided adjacent a workpiece to be sprayed. The orifice may be provided by a spray nozzle. Particulate matter is provided under pressure to the orifice as is an inert gas under pressure. The inert gases are provided under pressure so as to establish a static pressure at the orifice and provide a spray of particulate matter and gas onto the workpiece. The orifice is located in a region of ambient pressure which is substantially less than the static pressure at the orifice, to provide substantial acceleration of the spray of particulate matter and gas onto the workpiece. The inert gas may be heated before introduction into the orifice, preferably by exposing the gas to a temperature of 0xc2x0 C.-1000xc2x0 C. The static pressure at the orifice may be within a range of 1-20 atmospheres, and the region of low ambient pressure preferably has a pressure in the range of less than 1 atmosphere to 0.00001 atmosphere. The powder particles preferably have a size of 20-0.5 microns.
In accordance with the invention, the method may include the further step of recycling all of the inert gas from the workpiece, thereby conserving on the expensive inert gas which is typically used.
The providing of heated gas under pressure may be accomplished by providing a source of pressurized gas, coupling the source of pressurized gas to the nozzle or other object for providing the orifice, through a heater tube, and heating the heater tube to heat the gas. A flow of powder particles is introduced into the gas at one of a plurality of selected points of introduction along the heater tube and the nozzle as determined by an amount of desired heating of the powder particles before introduction at the nozzle, relative to the heating of the gas provided by the heater tube.
A cold spray gun in accordance with the invention includes an enclosed casing having a hollow interior, a spray nozzle mounted in a wall of the casing, a hollow coil mounted in the casing and coupled to the spray nozzle, a gas supply coupled to the hollow coil, a source of electrical power coupled to the hollow coil to provide heating thereof, and a powder feeder. A plurality of valves and injection ports are coupled to the powder feeder for delivering powder to one of various locations along the hollow coil and within the nozzle.
The enclosed casing may have a reflective interior surface so as to enhance the heating of the gas within the hollow coil. A pressure substantially lower than atmospheric pressure is established at the spray nozzle outside of the enclosed casing to provide substantial acceleration of the exiting particles and greatly enhance the coating formed on the workpiece.
The pressure substantially lower than atmospheric pressure established at the spray nozzle outside of the enclosed casing is preferably provided by an enclosed tank having the workpiece and the cold spraying gun mounted therein, in conjunction with a vacuum pump coupled to the tank. Whereas the cold spray gun has a nozzle with an orifice therein, and preferably a pressure of 1-20 atmospheres at the orifice, the pressure substantially lower than atmospheric pressure at the outside of the gun is preferably in the range of less than 1 atmosphere to 0.00001 atmosphere.
The enclosed tank may be coupled through a filter arrangement to a vacuum pump. The filter arrangement filters particulate matter from the overspray at the workpiece, and the vacuum pump produces the tank""s ambient pressure which is substantially less than atmospheric pressure. A compressor downstream of the vacuum pump compresses the gas from the workpiece which is drawn through the filter arrangement and through the vacuum pump, to provide compressed gas to the source of pressurized gas flow to the cold spray gun.
The powder flow may be provided by apparatus which includes an arrangement of valves and powder injection ports for introducing the powder flow at a selected one of a plurality of locations along the heating coil to provide a desired amount of heating of the powder flow before being sprayed by the cold spray gun onto the workpiece.